In recent years, the MPLS-TP (Multiprotocol Label Switching-Transport Profile) technology has been standardized by the IETF (Internet Engineering Task Force) as a new packet transport technology.
The MPLS-TP aims to realize a packet transport network most suitable for providing a packet service in the transport network based on SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) technology. The MPLS-TP is composed of a subset function extracted from the existing MPLS (Multiprotocol Label Switching) technology and a newly added function.
A network architecture of the MPLS-TP is composed of three Planes: Data-Plane (D-Plane), Management-Plane (M-Plane), and Control-Plane (C-Plane). In the M-Plane, monitor, control, and the like of a device and an LSP (Label Switch Path) of which the MPLS-TP network is composed are performed. In the C-Plane, set-up, maintenance, and the like of the LSP which is not controlled by the M-Plane are performed by a signaling between the devices. In the D-Plane, data transfer is performed according to a label switching process and an encapsulation process of the existing MPLS. Further, the D-Plane has a function for maintaining and managing a network that is called OAM (Operations, Administration, and Maintenance).
The MPLS-TP is an architecture in which these three Planes are logically or physically separated from each other and the D-Plane operates independently of the C/M-Planes. By using this architecture, the MPLS-TP removes dependency on an IP (Internet Protocol) layer and complexity, and ensures robustness required for the transport network. The OAM of the existing MPLS is dependent on the IP layer. Therefore, a new OAM function that is not dependent on the IP layer is added to the OAM function of the D-Plane of the MPLS-TP.
The MPLS-TP has a hierarchical structure of three layers. First, a transport layer of the MPLS-TP is called Transport Network Layer. The Transport Network Layer is configured by using the MPLS technology conforming to the IETF standard and a PWE3 (Pseudo Wire Emulation Edge to Edge) technology with which a virtual Point-to-Point path is formed on an MPLS network. The upper layer of the Transport Network Layer is called a Client Layer and various layer technologies can be applied to the Client Layer by the PWE3 technology. Further, the lower layer of the Transport Network Layer is called a Server Layer and in the Server Layer, the various layer technologies can be used for transmission between nodes in the MPLS-TP network like the existing MPLS.
In the MPLS-TP network that is composed of a device having the MPLS-TP function, by the PWE3 technology and the MPLS technology, two hierarchical end-to-end paths (PW path and LSP path) are established between LERs (Label Edge Routers) that are edge nodes of the network. A frame received from the Client Layer is encapsulated with two MPLS labels (PW label and LSP label) in two stages and transferred on the path. By performing the MPLS label stacking in a specific section of the path, hierarchization is additionally performed in the LSP path and whereby, monitor, maintenance, and the like of the section can be performed by the OAM function.
As mentioned above, when the hierarchization is additionally performed in the LSP path in the specific section of the path by the OAM function, and the monitoring and maintenance of the section is performed, a method for discovering an MTU (Maximum Transmission Unit) when transmitting the OAM frame remains as an issue. Further, the MTU is a maximum value of an amount of data that can be transferred at one time.
A packet transfer operation in a case mentioned above is shown in FIG. 16 and FIG. 17. In FIG. 16, a PW path 61 and an LSP path 41 are established between a communication device 10 and a communication device 30. An LSP path 81 for monitoring is further established in a section between a communication device 20 and the communication device 30 at a different hierarchical level. When a packet is transmitted in the section of the LSP path 81, a MPLS label is stacked. In this case, the MPLS label is a label indicating that the packet has passed through the LSP path 81 for monitoring.
An example of the packet transfer operation in a network shown in FIG. 16 is shown in FIG. 17. In FIG. 17, both the Client Layer and the Server Layer are Ethernet (registered trademark). It is assumed that the MTU of the data link between the communication device 10 and the communication device 20 is 1500 bytes and the MTU of the data link between the communication device 20 and the communication device 30 is also 1500 bytes.
First, in the communication device 10, by using the PWE3 technology, all the fields excluding an FCS (Frame Check Sequence) of an Ethernet frame received by the Client Layer are encapsulated with the PW label and the LSP label. It is assumed that the MTU of the Client Layer is set to 1478 bytes in consideration of the encapsulation.
Hereinafter, in a case in which the communication device 10 receives the Ether frame whose payload size is 1478 bytes, operation of the communication device 10 will be described in detail. First, the communication device 10 receives the Ether frame with a payload of 1478 bytes from the Client Layer (FIG. 17 (1)). The Ether frame is encapsulated with the PW label and the LSP label (FIG. 17 (2)). The Ether header is 14 bytes. The PW label and the LSP label are 4 bytes, respectively. Therefore, the Ether frame is 1500 bytes. The Ether header of the Server Layer is added to this Ether frame and the Ether frame with a payload of 1500 bytes is transferred (FIG. 17 (3)). The MTU between the communication device 10 and the communication device 20 is 1500 bytes. Therefore, this Ether frame can reach the communication device 20.
In the communication device 20, the Ether header of the Server Layer of the received Ether frame is removed. Next, it is recognized that a next forwarding destination of the Ether frame is the communication device 30. In order to forward the Ether frame to the communication device 30, the Ether frame has to pass through the LSP path 81 for monitoring. Therefore, the LSP label is further added (FIG. 17 (4)). Because this LSP label is 4 bytes, the Ether frame transmitted from the communication device 20 has the payload of 1504 bytes (FIG. 17 (5)).
Here, the MTU between the communication device 20 and the communication device 30 is 1500 bytes. Therefore, the Ether frame exceeds the MTU and whereby, the Ether frame cannot be transferred to the communication device 30.
Thus, it cannot be known by a transmission source node (in the above-mentioned example, the communication device 10) of the path that the frame size increases because of the excessive label stacking that is performed in the path beyond the expectation. Accordingly, a problem in which even it is not at the end of the path, the Ether frame that exceeds the MTU is transmitted and whereby, the end-to-end transfer of the Ether frame cannot be performed occurs.
In an IP layer, a Path MTU Discovery protocol described in patent literature 1 is used to solve a similar problem. The Path MTU Discovery protocol operates in the IP layer when a communication device (router) receives the frame that exceeds the MTU value. The communication device stores the appropriate MTU value in a destination unreachable notification (Destination Unreachable Message) of an ICMP (Internet Control Message Protocol) and sends it to the transmission source of the frame as a reply. When the communication device that is the transmission source of the frame receives the destination unreachable notification, it automatically performs correction so as to update the MTU value with the value described in the destination unreachable notification.
A technology with which a CC (Continuity Check) function of the Ether OAM recommended by IEEE 802.1ag or ITU-T Y.1731 is used is disclosed in patent literature 2. Specifically, a method in which a frame is transmitted while changing a size of a CCM (Continuity Check Message) frame by 1 byte for each frame and the communication device which receives the CCM checks the length of the reached/unreached frame is disclosed. By using this method, the length of the frame of which the end-to-end transfer cannot be performed can be detected.